Conventionally, a hardmetal (a WC—Co alloy or an alloy obtained by adding a carbonitride of Ti (titanium), Ta (tantalum), Nb (niobium), or the like to the WC—Co alloy) has been used for a cutting tool. With a growing tendency toward high-speed cutting in recent years, a hard alloy tool has more increasingly been used, the hard alloy tool being obtained by coating a surface of a base material such as a hardmetal, cermet or ceramics based on alumina or silicon nitride with a coated film composed of a carbide, a nitride, a carbonitride, a boronitride, and an oxide of IVa-, Va- and VIa-group metal in the element periodic table or Al (aluminum) to a thickness of 3 to 20 μm, with the use of CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition).
In particular, as coating by means of PVD can improve wear resistance without deteriorating strength of the base material, it is widely used for a cutting tool in which strength is required, such as a drill, an end mill, and a throw away tip for milling or turning.
Recently, in order to further improve efficiency in a cutting process, a cutting speed has been increased. With such a tendency, further wear resistance is required in the tool. If high wear resistance is required, however, toughness is lowered. Therefore, achievement of both high wear resistance and high toughness has been demanded.
In an attempt to meet this demand, a method of varying an internal stress such as compressive stress continuously or in a stepped manner in a coated film formed on a surface of the base material for the cutting tool has been proposed (Japanese Patent Laying-Open No. 2001-315006 (Patent Document 1)). Such a proposal has produced some effect in meeting the demand for achievement of both wear resistance and toughness.
In the cutting tool according to the above-mentioned proposal, the compressive stress of the coated film uniformly increases or decreases from a side of a surface of the coated film toward a side of the surface of the base material. Therefore, in order to considerably improve toughness, the compressive stress should be increased from the side of the surface of the base material toward the side of the surface of the coated film. Meanwhile, in order to considerably improve the wear resistance, the compressive stress should be increased from the side of the surface of the coated film toward the side of the surface of the base material.
In other words, if maximum compressive stress is attained at the surface of the coated film, toughness is excellent whereas wear resistance is poor. This is because the compressive stress uniformly decreases, continuously or in a stepped manner, toward the surface of the base material. In contrast, if maximum compressive stress is attained at the surface of the base material, wear resistance is excellent whereas toughness is poor. This is because the compressive stress uniformly decreases, continuously or in a stepped manner, toward the surface of the coated film.
In particular, in the cutting tool attaining the maximum compressive stress at the surface of the coated film, the coated film tends to self-destruct due to that large compressive stress after the coated film is formed (after coating is finished) or when impact stress is applied. Then, minute film peeling (hereinafter, referred to as film chipping) tends to occur, which adversely affects appearance of the cutting tool and cutting performance in high-precision process.
As achievement of both toughness and wear resistance in the cutting tool of this kind is one of the most basic characteristics, a cutting tool attaining both of these characteristics at a higher level has been desired.
Patent Document 1: Japanese Patent Laying-Open No. 2001-315006